Electric traction motor vehicles, such as locomotives, transit vehicles or off-highway vehicles, use power conversion systems to condition the electric power supplied to electric motors during propulsion and to control regenerative power from the motors during electrical retarding. If supplying DC motors, such a system will include an electric power "chopper" that is suitably controlled to vary the magnitude of load current and/or voltage as desired. Alternatively, in the case of alternating current (AC) motors, the system will include an electric power "inverter" that is suitably controlled to vary the amplitude and frequency of load voltage as desired. In either case, electric power flows from the DC source terminals to the load terminals of the controllable converter during "motoring" operation or in a reverse direction during "electrical braking".
In the electrical braking or retarding mode of operation of the power conversion system, the converter is so controlled that each motor acts as a generator driven by the inertia of the vehicle and supplies electric power which flows in a reverse direction through the converter and appears as direct and unipolarity voltage at the source terminals. As this electrical energy is used or dissipated, the traction motor(s) responds by absorbing kinetic energy and slowing the vehicle. Electrical braking is achieved by dynamic braking. Dynamic braking is effected by connecting a dynamic braking resistance between the DC source terminals. This resistance receives current from the converter, converts the electrical energy to thermal energy, and dissipates the resulting heat.
A power conversion system including a voltage source inverter for supplying AC traction motors is disclosed in U.S. Pat. No. 3,890,551--Plunkett, assigned to General Electric Company. A shunt capacitance (C) of a filter at the DC terminals of the inverter provides the "stiff" voltage required for proper operation of a voltage source inverter.
U.S. Pat. No. 4,093,900, assigned to General Electric Company, shows an exemplary dynamic brake circuit, although in the present state-of-the-art, it is preferable to replace the parallel array of separate braking resistors and their respectively associated electromechanical switches, as shown in U.S. Pat. No. 4,093,900, with a single bank of resistance elements connected to the DC link via an electric power chopper comprising a controllable solid-state electric valve that can be repetitively turned on and off in a pulse width modulation (PWM) mode to control the average magnitude of current in the resistor as desired. An example of this practice is disclosed in U.S. Pat. No. 4,761,600--D'Atre et .al., where the electric valve comprises a main thyristor for commutating the main SCR from a conducting state (on) to a non-conducting or current blocking state (off). Preferably, a solid-state gate turn-off device (GTO) could be substituted for the chopper shown in U.S. Pat. No. 4,761,600.
The filter capacitance means operates in conjunction with the electrical braking system. A more detailed description of the operation of an electrical braking system may be had by reference to U.S. Pat. No. 4,904,918--Bailey et al., issued Feb. 27, 1990 and assigned to the assignee of the present invention. During electrical braking, the capacitance means is called upon to attenuate transients generated by the operation of the chopper in varying the dynamic braking resistance so as to provide smooth braking effort.
In a typical system operated in a dynamic braking mode, the voltage appearing across the filter capacitance may be on the order of 1400 volts. During switching of the chopper circuit, the switching devices, such as GTO devices, must be capable of transitioning to a non-conducting state even though such high voltages are present. In a conventional system, a GTO device is protected by a snubber circuit which delays application of the full voltage to the GTO device until the device has sufficient time to reach a blocking state. Application of the full voltage prior to the GTO device reaching the blocking state may prevent the GTO device from turning off and result in an uncontrolled mode of operation. Further, the energy stored in the stray inductance in the dynamic braking circuit gets transferred to the snubber circuit during turn-off of the GTO device. The amount of this energy determines the peak voltage applied to the GTO device during turn-off. This voltage, along with the decay current in the GTO device, determines the switching losses in the GTO device.
It is obviously desirable to minimize the peak voltage and losses in the GTO device. One solution would be to keep the inductance between the filter capacitor and the GTO device at a low value. However, for practical reasons, the circuit inductance cannot be lowered sufficiently, particularly when additional elements such as brake cut-out switches are included in the circuit. Another possibility is to place capacitors in parallel circuit with the dynamic brake resistance and GTO switches. However, this solution produces circulating currents through the stray inductance, the DC filter or link capacitance, and the parallel capacitors, and for that reason is an impractical solution.